Mannose has been used as a material for mannitol synthesis and a constituent of animal cell culture medium. Further, it has been shown that mannose inhibits the growth of enterobacteria Salmonella, and its use as an additive for drinking water or feed for fowl such as chickens is being considered (R. H. Brown, in Foodstuff, June 12, vol. 10, 1989).
Furthermore, since mannose improves the taste of various types of food, its application as food additive has also been considered.
However, supply of mannose is scarce, since mannose is usually prepared by the acid-hydrolysis of glucomannan contained in plants such as wood and devil's tongue, or by processing glucose under high temperature in the presence of a molybdate catalyst; hence, mannose is extremely expensive.
On the other hand, the inventor of the present application has continued research in hope to establish a technique to produce mannose from fructose produced from glucose, which is in abundance and inexpensive, by using mannose isomerase, which converts mannose to fructose and vice versa, and a technique to produce mannose directly from glucose using mannose isomerase and glucose isomerase (an enzyme which converts D-glucose to fructose and vice versa) in combination.
Mannose isomerase is an enzyme, which was first found in 1956 by Palleroni, Doudoroff et al., in Pseudomonas saccharophila (J. Biol. Chem., vol. 218, p. 535, 1956). Later, the inventor of the present application and his coworkers found and reported that a bacterium identified as Xanthomonas ruburilineans also produces mannose isomerase (J. Agr. Chem. Soc. Japan, 37, 524, 1963; Agric. Biol. Chem. 28, 601, 1964). Also, the inventor has found and reported that a Streptomyces bacterium produces the same enzyme (Report of the Fermentation Research Institute Agency of Industrial Science and Technology, 28, 89, 1966).
However, all of the above conventional mannose isomerases have low optimum temperatures in the range of 35° C. to 40° C., thereby being poor in thermostability and problematic for industrial use.
In relation to this point, the inventor of the present application has invented a thermostable mannose isomerase, which is produced by a Pseudomonas bacterium, and filed a Japanese patent application (Japanese Patent Provisional Publication No. 218370/1992).
However, in order to produce mannose from fructose, for example, by using the above-mentioned mannose isomerase as an immobilized enzyme, or to produce mannose from glucose by using glucose isomerase in combination, a mannose isomerase which may be used consistently at a temperature in the range of 55° C. to 60° C. for a prolonged period of time, was required.
The present application addresses the above-described circumstances, and its object is to provide a novel thermostable mannose isomerase that shows excellent stability at high temperature compared to conventional enzymes, and stable under constant use at temperatures in the range of 55° C. to 60° C. for a prolonged period of time.
Further, the present application also provides a method for producing the said thermostable mannose isomerase and a method for producing mannose by using the same.